Patricia M. Frontini

Development of bicontinuous morphologies in polysulfone-epoxy blends

The development of bicontinuous morphologies in 10 wt% polysulfone (PSu)‐epoxy (DGEBA)/anhydride (MTHPA) blends, was followed by optical and scanning electron microscopy. Blends cured at 808C revealed the formation of large epoxy-rich domains surrounded by a PSu-rich matrix, soon after the cloud point. Advancing the cure led to an increase in the volume fraction and the coalescence of epoxyrich domains. A bicontinuous primary morphology was thus generated. A secondary phase separation was observed in both primary phases from the very beginning of the phase-separation process. While spinodal demixing was clearly the mechanism by which the primary morphology was generated, nucleation-growth could be responsible of the secondary phase separation. Postcure steps produced a change in the composition of phases as revealed by DMA, and in the secondary morphology as observed by SEM. A postcure at 1208C led to a single Tg at 1158C with a small shoulder at higher temperatures. A postcure at 2008C led to a Tg at 1088C for the epoxy-rich phase and a Tg at 1378C for the PSu-rich phase. The partial purification of the thermoplastic phase produced a significant enhancement of toughness.KIC was increased from 0.65 MPa m 1/2 for the neat thermoset to 1.10 MPa m 1/2 for the blend postcured at 2008C. q 1999 Elsevier Science Ltd.

Liquid rubber modified vinyl ester resins: fracture and mechanical behavior

Abstract The fracture and mechanical behavior of vinyl ester resins (DVER) cured with styrene (S) and modified with two different liquid rubbers has been determined and related to the microstructure of the resulting modified thermosets. Carboxyl terminated poly(butadiene-co-acrylonitrile) (CTBN), a common toughening agent for epoxy resins, is an almost unreactive rubber with the DVER and S comonomers. During crosslinking the system undergoes a phase separation mechanism similar to that occurring in unsaturated polyester resins (UPE) modified with a low profile additive (LPA), such as polyvinyl acetate (PVAc). This process leads to materials, which exhibit a sharp drop in density at high CTBN concentrations (≥10% by weight) and to the development of a co-continuous microstructure in these materials. This feature is consistent with a maximum in fracture toughness as a function of the additive (CTBN) content, followed by a rapid deterioration in toughness at higher concentrations. On the other hand, the use of a reactive rubber, vinyl terminated poly(butadiene-co-acrylonitrile, VTBN, as the additive leads to a different morphology consisting on rubber inclusions in the thermoset matrix. This structure gradually reduces the fracture and mechanical performance of the resins modified with increasing concentration of reactive elastomer.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems. 2. Morphologies generated and resulting properties

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out in situ , during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.

The application of the essential work of fracture methodology to the plane strain fracture of ABS 3-point bend specimens

The applicability of the EWF methodology to 3-point bend (SEB) specimens under conditions other than plane stress has been assessed experimentally. Different fracture conditions, pure plane strain and plane strain/plane stress transition, were obtained by varying the specimen thickness and testing temperature (20 and 80 8C). Post-mortem fracture surfaces appeared always completely stress-whitened, indicating ductile fracture. The load– line displacement plots are similar over a well-defined range of ligament lengths for which the application of the EWF methodology was in principle possible. Nevertheless, in experiments conducted at room temperature, crack growth was observed to initiate before maximum load and complete ligament yielding. This behaviour was confirmed through plastic collapse analyses. A critical ligament length was found, over which the total specific work of fracture was dominated by edge effects. Below this critical ligament length, EWF methodology was still applicable and it was possible to extrapolate reliable wIe values. q 2002 Elsevier Science Ltd. All rights reserved.

The use of load separation criterion and normalization method in ductile fracture characterization of thermoplastic polymers

Load separation is the theoretical basis for the single-specimen J-integral experiment and the incremental calculation of J-integral crack growth resistance (J-R) curves. This criterion has been experimentally studied in nongrowing crack records in several materials, and more recently a new method to extend the applicability to growing crack experiments has been proposed in testing steel. This article examines the applicability of the load separation criterion for evaluating ductile fracture mechanics parameters in rubber-modified polystyrenes and thermally treated polypropylene in the bending configuration. This criterion allows the load to be represented as the multiplication of two independent functions: a material deformation function and a crack geometry function. Its validity is evaluated with both stationary and growing crack experiments. η-factor calculation for smooth and side-grooved specimens was also tried using the simple method of Sharobeam and Landes, in order to identify material dependency. This article also investigates the applicability of the normalization method, based on the load separation criterion for evaluating J-R curves on PP and PS. A simple approach which combines a blunt notched and a precracked specimen experiment is proposed to determine the J-R curve of the materials studied. The resulting J-R curves are compared with multiple specimen results available in the literature for these materials. A good agreement between the J-R curves obtained from this simple method and from the multiple specimen technique was found.

A simple method for J-R curve determination in ABS polymers

Abstract The objective of this paper is to investigate the applicability of a very simple new method, based on load separation criterion, developed for evaluating J-R curves in steels, when applied to ABS polymers. This technique proposes the testing of a single precracked specimen and a single blunt notched specimen. The latter provides a method to detect the plastic displacement range during crack tip blunting in which the assumption of load separation criterion in the precracked specimen is valid, and also allows determination of J IC independently of J-R curve determination. The deformation material function is assumed to be a Ramberg-Osgood type law deformation function which leads to a very easy procedure to determine J-R curves. The J-R curve resulting from this new method is compared with one resulting from a traditional multiple specimen J-R curve determination method. Both results are in good agreement.

Mechanical and fracture behaviour evaluation of commercial acrylic bone cements

The deformation and fracture behaviour of some commercial acrylic bone cements have been investigated. Cements were characterized by gel permeation chromatography, dynamic mechanical analysis and scanning electron microscopy. The influence of liquid to powder ratio, curing temperature, strain rate and non-reacted monomer was analysed for one radiolucent cement. Results showed that the β transition activation process influences both deformation and fracture behaviour. Fracture surface stress whiteness revealed the presence of crazes as the main plastic deformation mechanism. Non-reacted monomer acted as a plasticizer leading to materials with lower yield strength, σy, that induces crack tip blunting and improves toughness. It appears that the presence of radiopacifier fillers also improves fracture toughness by promoting interactions between the crack and the second phase dispersion.

Fracture toughness determination of ABS polymers using the J-method

Abstract The suitability of J–R curve fitting methods proposed by ASTM E 813-81 and E 813-87 metal standards in fitting ABS J–R curves is discussed. Tests were carried out at room temperature on one commercial grade ABS resin in three-point bending. Specimens were precracked with a razor blade and tests were performed over a X20 range of crosshead rate in the quasistatic regime. Specimens of different geometric relationships (B/W, a/W, smooth specimens of different thickness, and side-grooved specimens) were assayed. The two ASTM fitting procedures were also checked against data reported by other authors for other ABS type resins. Model appropriateness was checked by statistical analysis. Results appeared to be geometrically independent for deeply notched specimens. No significant variation in the J–R curve was found for changes in the displacement rate. Both J–R curve fittings appeared to be adequate. The ASTM E 813-87 procedure led to less conservative critical initiation J IC values.

Blends of epoxy/anhydride thermosets with a high-molar-mass poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA, M n =232 000) was used to modify a stoichiometric epoxy (diglycidyl ether of bisphenol A; DGEBA)-anhydride (methyl tetrahydrophthalic anhydride; MTHPA) thermoset. PMMA concentrations in the range 3-7 wt% led to morphologies consisting of a continuous PMMA-rich region that appeared rough and striated in scanning electron micrographs and large domains of the thermoset exhibiting a dispersion of PMMA-rich particles in the micrometre range. These morphologies are the result of the critical point location, estimated at 2. 1 wt% PMMA as a result of the high molar mass of the additive. A 5 wt% PMMA led to an increase of the stress intensity factor K IC from 0.65 MPa m 1/2 , for the neat thermoset, to 0.94MPa m 1/2 . However, T g was reduced from 117 °C for the neat thermoset to about 105 °C for the PMMA-modified material. The T g decrease is ascribed to the differential segregation of both monomers to the PMMA-rich phase. No influence of PMMA addition on the cure kinetics was observed. An upper critical solution temperature was observed, meaning that cloud-point conversions increased with cure temperature. Phase separation took place before gelation in the temperature range investigated in this study.

Fracture characterization of low‐density polyethylenes by the essential work of fracture: Changes induced by thermal treatments and testing temperature

The tensile deformation and fracture behavior of commercially available low-density polyethylene (LDPE) films, having different molecular characteristics, was studied. Submitting samples to specific thermal histories controlled the morphological structure of these semicrystalline polymers. Phase-structure analysis of the resulting materials was performed by DMA and DSC analyses. The plane-stress essential work of fracture methodology was chosen because the materials used had failed after complete necking of the remaining ligament. Significant differences in behavior, induced by thermal treatments, were found for the tensile yield stress and the specific nonessential work of fracture, but not in the specific essential work of fracture. The results show that the mechanical properties and fracture behavior depend not only on the crystallinity levels and molecular weight characteristics of the samples, but also upon the degree of structural continuity. The β-relaxation process, associated with the crystal-amorphous interphase, strongly influences the fracture behavior at testing temperatures chosen below the β-relaxation temperature.